This commit adds the implementation for an "isc_dnsstream_assembler_t"
object. The object is built on top of "isc_dnsbuffer_t" and is
intended to encapsulate the state machine used for handling DNS
messages received in the format used for messages transmitted over
TCP.
The idea is that the object accepts the input data received from a
socket, tries to assemble DNS messages from the incoming data and
calls the callback which contains the status of the incoming data as
well as a pointer to the memory region referencing the data of the
assembled message. It is capable of assembling DNS messages no matter
how torn apart they are when sent over network.
The following statuses might be passed to the callback:
* ISC_R_SUCCESS - a message has been successfully assembled;
* ISC_R_NOMORE - not enough data has been processed to assemble a
message;
* ISC_R_RANGE - there was an attempt to process a zero-sized DNS
message (someone attempts to send us junk data).
One could say that the object replaces the implementation of
"isc__nm_*_processbuffer()" functions used by the old TCP DNS and TLS
DNS transports with a better defined state machine completely
decoupled from the networking code itself.
Such a design makes it trivial to write unit tests for it, leading to
better verification of its correctness.
Another important difference is directly related to the fact that it
is built on top of "isc_dnsbuffer_t", which tries to manage memory in
a smart way. In particular:
* It tries to use a static buffer for smaller messages, reducing
pressure on the memory manager (hot path);
* When allocating dynamic memory for larger messages, it tries to
allocate memory conservatively (generic path).
These characteristics is a significant upgrade over the older logic
where a 64KB(+2 bytes) buffer was allocated from dynamic memory
regardless of the fact if we need a buffer this large or not. That is,
lesser memory usage is expected in a generic case for DNS transports
built on top of "isc_dnsstream_assembler_t."
338cf3e467